Field of the Invention
The invention relates to a method for correctly geometrically assigning x-ray images of a patient.
During the operative treatment of fractures of long hollow bones, care must be taken to ensure that the corresponding extremity axes of the patient are reproduced in their original form. For instance, with a femur fracture, the leg axis (avoiding varus or valgus position), the leg rotation and the leg length must be set correctly.
It is known to visually monitor the leg axis intra-operatively by an auxiliary device. To this end, the cable of an electrocautery is often used. It is also known to measure the leg length in comparison with the healthy leg of the patient and to visually align the leg rotation with the healthy leg.
An imaging device, e.g. an x-ray C-arm is also often intra-operatively available. X-ray devices of this type mostly contain a comparably small field of view of for instance only 15 cm diameter. A long hollow bone is considerably longer so that several x-ray images have to be recorded in order to image the whole bone. More recent methods enable the correct geometrical composition of individual x-ray recordings to form an overall image, from which the leg axis and the leg length can then be derived.
“Correct geometrical” means that with respect to the imaged patients, the images which are not related per se with respect to an imaginary overall image are in a correct position, in other words, are assigned to one another like image sections which are in a fixed position relative to one another. The real ratios on the patient are therefore imaged in a correct position on account of their opposite position and orientation.
Two approaches exist in order to compose the images: with the first approach, a very large number of x-ray images are recorded in pairs in each instance with a large image overlap in order therefrom to generate a coherent image. With a second approach, x-ray images are recorded with no overlap, e.g. an x-ray image of the hip, one of the knee and one of the ankle is recorded. On account of the absent overlap, a uniform reference system must nevertheless be created here for all (partial) images in order to correctly geometrically arrange and/or relate the individual images. The respective position determination of the imaging system would be conceivable here for instance on the basis of a conventional tracking system.
Alternatively, the article “‘Image Fusion for Intra-operative Control of Axis in Long Bone Fracture Treatment’, Peter Messmer et al., European Journal of Trauma 2006, No. 6, P. 555-561, Urban & Vogel” discloses a rigid marker plate with x-ray visible markers (location codes). The marker plate is placed under the patient so that parts of the same are visible in all partial images. The marker plate enables a correct geometrical assignment of the individual images to one another since corresponding x-ray visible location codes of the marker plate are visible in the x-ray image.
With the use of a marker or code plate of this type, the relevant x-ray markers are however always visible in the x-ray images in an interfering manner. The marker plate must also be positioned prior to supporting the patient, a subsequent positioning is no longer possible since this must be placed under the patient. The marker plate must be stored and also cleaned in the operating room. In some circumstances, the marker plate pushes through the patient rest in an interfering manner.